The present invention relates to a magnetic resonance imaging system, and more particularly, to a magnetic resonance imaging system having an advanced magnetic gradient field coil.
A magnetic resonance imaging (MRI) system for examining a tissue type of the human body applies a gradient field of varying intensities to the human body, to thereby display the arrangement of nuclear spins of bodily tissue. That is, when a radio frequency (RF) pulse wave within a strong static magnetic field is applied to the human body, the nuclear spins of bodily tissue are excited and magnetic resonance (MR) signals are generated when the gradient magnetic field appropriate for bodily tissue is applied. By image-processing the above magnetic resonance signals, the type of the bodily tissue can be examined.
FIG. 1 is a schematic diagram showing a conventional magnetic resonance imaging (MRI) system. As shown in FIG. 1, the conventional MRI system is provided with a computer 11 for performing overall control and signal processing, a pulse generator 12 for generating RF pulse signals and gradient pulse signals having variable intensities, according to control signals of the computer 11, a main magnet 13 having a C-type structure and for generating a static magnetic field B.sub.0, a disk-shaped gradient magnetic coil 14 attached to two magnetic-polar surfaces of the main magnet 13 and for generating a gradient field according to the gradient field pulse signals, an RF coil 15 for emitting oscillation waves according to the RF pulse signal to an imaging object and receiving MR signals from the imaging object, and an analog-to-digital converter 16 for converting the MR signals from the RF coil 15 into digital signals and outputting the converted digital signals to the computer 11.
The RF coil 15 can be divided into a transmitting portion (not shown) and a receiving portion (not shown). The region represented by the slashed oval between the upper and lower gradient field coils 14 represents a region for an imaging object, for example, the region where a patient is positioned. In the region, the static magnetic field B.sub.0 from the main magnet 13 and the gradient field from the gradient field coil 14 are applied. Here, the gradient field coils 14 are provided in pairs with respect to each of the X-, Y- and Z-axes, respectively.
The pulse generator 12 generates RF pulse signals and gradient field pulses with respect to the three pairs of gradient magnetic coil 14. Accordingly, the transmitting portion of the RF coil 15 transmits a RF magnetic field according to the RF pulse signal to the imaging object, and the gradient field coils 14 generate a gradient field according to the gradient field pulse signals. Also, the receiving portion of the RF coil 15 receives the MR signals from the imaging object and then outputs the received MR signals to the analog-to-digital converter 16. When the computer 11 processes the MR data from the analog-to-digital converter 16, the arrangement of the nuclear spins of bodily tissue can be transformed into the MR image.
FIG. 2 is an example of the X-axis gradient field coil of FIG. 1. Here, reference numeral 141 denotes an X-axis upper gradient field coil and reference numeral 142 denotes an X-axis lower gradient field coil. The X-axis upper and lower gradient field coils 141 and 142 have a disc-like structure. The Y-axis gradient field coils (not shown) also have a disc-like structure. That is, the Y-axis upper and lower gradient field coils are rotated 90.degree. with respect to the X-axis upper and lower gradient field coils 141 and 142 around the Z-axis. The Z-axis upper and lower gradient field coils (not shown) are formed of concentric-circular coils on the disc of the X-axis upper and lower gradient field coils 141 and 142. Here, current directions of the Z-axis upper and lower gradient field coils oppose each other.
As shown in FIG. 2, the upper gradient field coil 141 and the lower gradient field coil 142 consist of two semi-disc-like coils, respectively. That is, the gradient field coil of one axis has four semi-disc-like coils. Here, they are connected in parallel and/or in series, to thereby be supplied with gradient field pulse from the pulse generator 12 of FIG. 1. Each semi-disc-like coil is divided into a gradient field portion of many strands, which cross the inside of the semicircle, and a reverse circulation portion located along the perimeter of the semicircle. The currents of strands of the gradient field portion meet with that of the reverse circulation portion. Here, the gradient field generated from the gradient field portion of each semi-disc-like coil enlarges a linear gradient field of the image region. However, a reverse magnetic field generated from the reverse circulation portion reduces the linear gradient field. The magnetic field direction of the linear gradient field is the same as that of the static magnetic field B.sub.0 of FIG. 1. Accordingly, enlarging/reducing the linear gradient field represents actual enlargement/reduction of the image region.
In the above conventional MRI system, since the gradient field coil is disc-shaped, the strong influence of the reverse magnetic field reduces the linear gradient field and the image region thereof. Also, in order to enlarge the image region, a polar surface of the main magnet should be enlarged, increasing cost. Furthermore, vibration and acoustic noise occurring between the gradient field coil and the magnetic pole of the main magnet is severe.